This invention relates in general to high alloy steel that is subjected to cyclic loading and more particularly to a process for creating on a surface of a machine component made from high alloy steel a glaze having a refined microstructure that enables the component to better withstand cyclic loading and to a machine component having such a glaze.
The typical antifriction bearing in its most basic configuration has two races, each provided with a raceway, and rolling elements which are located between the races and roll along the raceways when one of the races rotates relative to the other race. Rolling, as opposed to sliding, which is characteristic of sleeve bearings, produces relatively little friction, and hence bearings with rolling elements are aptly called antifriction bearings. These bearings assume several configurations. In some the rolling elements are balls and the raceways are concave to conform to the curvature of the balls. Others have cylindrical rollers, while still others have tapered rollers.
Irrespective of their configuration, the rolling elements concentrate the load in the limited areas where the contact exists. The raceways must withstand these loads without permanent deformation and further must experience very little wear. To this end the races along their raceways are quite hard. Apart from that, the rolling elements apply cyclic stresses to the raceways, so the races along their raceways must likewise resist spalling which tends to occur in the presence of cyclic stresses. In other words, the races must have an acceptable life.
For most applications requiring antifriction bearings, plain carbon steels serve well for the races and the rolling elements of those bearings. Some manufactures use low carbon alloy steels which, once machined to the proper configuration, are case-hardened and heat treated to provide the hard wear-resistant surfaces. Others use a tempered steel with a greater carbon content, and induction harden that steel along the raceways and the surfaces of the rolling elements. Still others use through-hardened steel, likewise of greater carbon content. Irrespective of the carbon content of the steels, the races along their raceways and the rolling elements along their exterior surfaces are hard enough to resist deformation and withstand wear. Moreover, heat treatments leave the microstructure along the critical surfaces of the races and rollers with small carbides--mostly cementite or iron carbide--so the bearing will have an acceptable fatigue life.
Some bearings, however, operate under extremely hostile and severe conditions that require better steels--indeed, steels which will withstand longer cyclic loading at higher loads and better resist wear. For these bearings, manufacturers have turned to high alloy steels, such as high speed steels or high carbon stainless steels. Certain alloying elements, such as chromium, in these steels unite with carbon to form carbides, and these carbides precipitate quite early in the transition from the molten state. They consolidate as the cooling continues to form rather large particles. Ingots of any steel cool slowly, and when ingots of high alloy steel cool, the carbides of chromium and other alloying metals grow, indeed, considerably larger than the particles of cementite or iron carbide one also finds in plain carbon steel. These carbides prevent the manufacturers from acquiring finely ground finishes along the raceways of the races, and this in turn degrades the fatigue performance of the bearings.
To be sure, procedures exist to reduce the size of carbides in steel, but these procedures have disadvantages. For example, rapid solidification will keep the carbides small, but ingots are much too massive to cool quickly enough too prevent the growth of large carbide particles. Also the particle size of carbides remains small when the bearing components are formed from powdered metal, but powder metallurgical procedures are quite costly.
Bearings do not represent the only machine components which experience cyclic stresses along critical surfaces--stresses which are conducive to spellings and fatigue failure. Gears, traction drives, and cams likewise see such loading, and along critical surface areas they should have a fine microstructure that is devoid of large carbide particles.
The present invention resides in a machine component that is formed from a high alloy steel, such as high speed steel or high carbon stainless steel, and operates under conditions which subject it to cyclic loading along one of its surfaces. The steel contains large carbide particles in its core, but along the surface that is subject to the cyclic loading, it has a glaze in which the carbide particles are quite small and the microstructure is otherwise quite fine--indeed, a dendritic network. The fine microstructure of the glaze retards spalling and thus extends the fatigue life of the component. To derive the glaze, a high energy beam is directed at the surface in the presence of an inert gas to momentarily melt the surface. The molten surface layer freezes almost instantaneously--or in other words is self-quenched--and as a consequence the carbides exist in the glaze at a much smaller size.
U.S. Pat. No. 3,737,565 considers the interaction of a high energy beam with a bearing surface. But the process of the present invention is unique and the results obtained from employing this invention are quite different from those derived from the process of U.S. Pat. No. 3,737,565. The process of that patent actually removes something from the steel to make a section of the steel cleaner. In particular, the melting is performed in a vacuum, so the total quantity of elements such as sulfur and aluminum is greatly reduced. The melting and rapid solidification utilized by the process of the present invention are performed at atmospheric pressure. An inert shielding gas prevents oxidation of the liquid. On utilizing the process, the chemical components within the steel are redistributed within the resulting glaze, but the chemical composition of the steel is not changed. The process is concerned with high speed steels and high carbon, high chromium stainless steel. By employing the process of the present invention a very fine dendritic microstructure is created in the glazed material. In addition, the very large alloy carbides in the original steel are greatly reduced in size in the glazed zone. Even after subsequent heat treating procedures, the alloy carbides within the glaze are small compared to the carbides in the core. The improved fatigue life resulting from the invention is due to a reduction in the size of the grains and alloy carbide particles and not the removal of any constituents or chemical species from the steel.